1. Technical Field of the Invention
The present invention relates to non-mechanical optical path switching and its application to dual beam spectroscopy including gas filter correlation radiometry.
2. Discussion of the Related Art
Non-mechanical optical path switching has many potential applications particularly in the field of dual beam spectroscopy. In dual beam spectroscopy, light from a radiation source is divided between two optical paths. Each optical path generally contains some medium through which the radiation is transmitted and thus partially absorbed. The key measurement in this type of spectroscopy is related to the difference in optical transmission between the two paths for the incident radiation. Although the gas filter correlation radiometer (GFCR) is but one example of a dual beam spectrometer, for illustrative purposes this application will be discussed in detail.
Gas filter correlation radiometers (GFCRs) may remotely infer the concentration of a gas species along some external path. In many GFCRs gas sensing is accomplished by viewing through two optical cells the emission/absorption of the gas molecules along the external path. These two optical cells, often called the correlation and vacuum cells, correspond to the media found in the two optical paths of a dual beam spectrometer. The correlation cell contains a high optical depth of a gas species i and thus strongly absorbs radiation at the molecular transition wavelengths of the particular gas. In effect the correlation cell acts as a spectral "notch filter", the spectral notches being coincident with the band structure of gas species i. On the other hand, the vacuum cell encloses either a vacuum or a gas that is non-absorbing in the spectral region of interest. The difference signal .DELTA.S between these two views of the emitting/absorbing gas species i can be related to the concentration of this gas along a path external to the GFCR.
In one known GFCR shown in FIG. 1, the radiation is divided by a simple beam splitter P1 into two paths--one containing a correlation cell P2 and the other a vacuum cell P3. The difference signal .DELTA.S is derived from the difference in the outputs of two detectors P4 and P5 respectively located at the end of the two optical paths. Suitable collecting optics P6 and an interference filter P7 are provided to collect and spectrally filter the light. In other GFCRs, the critical .DELTA.S is derived from a single detector P8, as shown in FIG. 2. In this case, the .DELTA.S is generated by physically moving the correlation and vacuum cells P2 and P3 periodically in front of the GFCR detector. This is often accomplished by mounting the correlation and vacuum cells on a rapidly rotating wheel P9 so that they alternately pass in and out of the single optical path.
The first GFCR approach of FIG. 1 uses two detectors to derive the small .DELTA.S signal. GFCR measurements of gas concentration are extremely sensitive to minute drifts in the response of the two detectors which must be frequently and precisely balanced. Also, surface response inhomogeneities on the two detectors can lead to significant measurement errors if the radiation background being viewed by the GFCR is spatially inhomogeneous. Such a situation may occur, for example, in an aircraft or satellite nadir viewing application.
The single detector approach of FIG. 2 has the disadvantage that some mechanical means must be used to alternate the view of the detector through the correlation and vacuum cells. Although this mechanical optical path switching may occur rapidly on the order of 100 Hz, it may nevertheless limit the GFCR performance in certain demanding applications where the background radiation varies rapidly, e.g., in some nadir earth-viewing cases. In applications where instrument maintenance is restricted or impossible, such as satellite applications, the presence of rapidly moving instrument components is an important constraint as it may limit operational lifetime. Angular momentum compensation must also be addressed especially for satellite applications.